This invention relates to the treatment of a combustible gas containing radioactive Kr-85, which is generated in a nuclear reactor and a fuel recycling plant.
One of the methods of separating and recovering Kr-85 from a gas containing Kr-85, fission product gas, which is generated in a nuclear power plant or the like is the cryogenic distillation method including a distillation column wherein the gas is treated by a cryogenic distillation processing and the contained gases are separated by the utilization of the differences in boiling point. In the case of seperating and recovering Kr-85 by said cryogenic distillation method, oxygen which might be contained in the gas would be condensed with acetylene generated during the cryogenic distillation processing, and have a danger of catching fire and exploding because of such an inflammable source as static electricity.
Heretofore, a following measure has been taken to solve the above problem. That is, firstly, hydrocarbons such as acetylene are removed from the waste gas by burning; thereafter oxygen contained in the gas is diluted; hydrogen is added to the gas and said oxygen is reduced to water while passing through the medium of molecular sieves; thereafter solid impurities such as dusts are removed; a part of the gas after the removal of the impurities is circulated as a diluent gas for the oxygen; the rest of the gas is used for adsorbing and separating Kr-85 in a Kr-85 separating, recovering means packed therein with activated charcoal; the clean gas after the separation is discharged to the atmosphere; and the rest of the gas containing Kr-85, afer discharging, is recovered to a storage vessel after condensation.
However, the waste gas from a nuclear reactor and a fuel recycling plant normally contains 20 to 50% oxygen, and the following problems are presented in the case of removing said oxygen by the conventional reduction method of oxygen by adding hydrogen.
1. Consumption of hydrogen is increased and cost of operation is extremely high. (For example, when a waste gas containing oxygen of 50% concentration is treated by 25 Nm.sup.3 /h, hydrogen consumption reaches 600 Nm.sup.3 /h.)
2. The addition of hydrogen and control in the dilution of oxygen are difficult, particularly the stabilities are low with respect to gas flow rate and with respect to the fluctuation in concentration of oxygen after the removal of hydrogen by the reduction of oxygen, and there usually exists a danger of the explosion of hydrogen due to a possible error in operation.
3. As the flow rate of the circulating gas for diluting oxygen amounts of 10 to 25 times the flow rate of the gas to be treated, the volume of the equipment becomes extremely larger.